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  流 Stream
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  <h1 id="流-Stream"><a href="#流-Stream" class="headerlink" title="流 Stream"></a>流 Stream</h1><hr>
<h2 id="基础概念"><a href="#基础概念" class="headerlink" title="基础概念"></a>基础概念</h2><h3 id="流"><a href="#流" class="headerlink" title="流"></a>流</h3><p>流处理是对运动中的数据的处理，在生成或接收数据时直接计算数据。应用程序中分析和查询不断存在，数据不断地流经它们。在从流中接收到事件时，流处理应用程序对该事件作出反应。</p>
<p>如果我们使用传统的循环迭代方式对数据集进行复杂计算，常常会带来两个弊端：</p>
<ol>
<li>迭代次数多，迭代次数跟函数调用的次数相等。</li>
<li>频繁产生中间结果，存储开销无法接受。</li>
</ol>
<p>流处理可以立即对事件做出反应，且可以处理比其他数据处理系统大得多的数据量：直接处理事件流，并且只保留数据中有意义的子集。尤其是面对持续生成，本质上是无穷尽的数据集。</p>
<h3 id="Java-Stream-类"><a href="#Java-Stream-类" class="headerlink" title="Java Stream 类"></a>Java Stream 类</h3><p>JDK 1.8 新增。将要处理的元素集合看作一种流，在管道的节点上进行处理。使代码更简洁易读。</p>
<p>集合接口有两个方法来生成流，数据类型将由 Collection 转化为 Stream 。</p>
<ul>
<li><code>stream</code> 方法：为集合创建串行流。</li>
<li><code>parallelStream</code> 方法：为集合创建并行流。</li>
</ul>
<ol>
<li><p>Stream 的遍历方式和结果与 Iterator 无差别（便于转化），其优势在于其原型链的设计使得它可以对遍历处理后的数据进行再处理。</p>
</li>
<li><p>parallelStream 提供了流的并行处理，底层使用 Fork/Join 框架，简单理解就是多线程异步任务的一种实现。处理过程中会有多个线程处理元素，具体由 JDK 负责管理。不保证有序性。</p>
</li>
<li><p>串行流和并行流之间可以通过 <code>parallel</code> 和 <code>sequential</code> 方法相互转化。</p>
</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Stream&lt;Integer&gt; stream = list.stream();                     <span class="comment">// 声明作为流处理</span></span><br><span class="line">ParellerStream&lt;Integer&gt; pStream = stream.parallel();        <span class="comment">// 转化为并行流</span></span><br></pre></td></tr></table></figure>

<h3 id="流操作"><a href="#流操作" class="headerlink" title="流操作"></a>流操作</h3><p>流处理的每个操作阶段都会封装到一个 Sink 接口里，处理数据后再将数据传递给下游的 Sink。</p>
<p>Stream 上的所有操作分为两类：中间操作和结束操作。Stream 是延迟执行的，只有调用到结束操作，才触发整个流水线的执行。如果未定义结束操作，那么流处理什么也不会做。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 获取空字符串的数量</span></span><br><span class="line"><span class="type">int</span> <span class="variable">count</span> <span class="operator">=</span> strings.parallelStream()                       <span class="comment">// 声明作为流处理</span></span><br><span class="line">                   .filter(string -&gt; string.isEmpty())     <span class="comment">// 中间操作，过滤空元素</span></span><br><span class="line">                   .count();                               <span class="comment">// 结束操作，计数</span></span><br></pre></td></tr></table></figure>

<hr>
<h2 id="中间操作"><a href="#中间操作" class="headerlink" title="中间操作"></a>中间操作</h2><h3 id="映射-map"><a href="#映射-map" class="headerlink" title="映射 map"></a>映射 map</h3><p><code>map</code> 方法用于映射每个元素到对应的结果，其实就是对结果进行转化。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 获取对应的平方数</span></span><br><span class="line">List&lt;Integer&gt; numbers = Arrays.asList(<span class="number">3</span>, <span class="number">2</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">7</span>, <span class="number">3</span>, <span class="number">5</span>);</span><br><span class="line">List&lt;Integer&gt; squaresList = numbers.stream().map( i -&gt; i*i).distinct().collect(Collectors.toList());</span><br></pre></td></tr></table></figure>

<h3 id="过滤-filter"><a href="#过滤-filter" class="headerlink" title="过滤 filter"></a>过滤 filter</h3><p><code>filter</code> 方法用于通过设置的条件过滤出元素。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">List&lt;String&gt;strings = Arrays.asList(<span class="string">&quot;abc&quot;</span>, <span class="string">&quot;&quot;</span>, <span class="string">&quot;bc&quot;</span>, <span class="string">&quot;efg&quot;</span>, <span class="string">&quot;abcd&quot;</span>,<span class="string">&quot;&quot;</span>, <span class="string">&quot;jkl&quot;</span>);</span><br><span class="line"><span class="comment">// 获取空字符串的数量</span></span><br><span class="line"><span class="type">long</span> <span class="variable">count</span> <span class="operator">=</span> strings.stream().filter(string -&gt; string.isEmpty()).count();</span><br></pre></td></tr></table></figure>

<h3 id="筛选-limit-skip"><a href="#筛选-limit-skip" class="headerlink" title="筛选 limit/skip"></a>筛选 limit/skip</h3><p><code>limit</code> 方法用于获取指定数量的流(前 n 个)， <code>skip</code> 方法用于去除指定数量的流(前 n 个)。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 筛选出 11-20 条数据</span></span><br><span class="line"><span class="type">Random</span> <span class="variable">random</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">Random</span>();</span><br><span class="line">random.ints().limit(<span class="number">20</span>).skip(<span class="number">10</span>).forEach(System.out::println);</span><br></pre></td></tr></table></figure>

<h3 id="排序-sorted"><a href="#排序-sorted" class="headerlink" title="排序 sorted"></a>排序 sorted</h3><p><code>sorted</code> 方法通过 Comparable 接口对流进行排序，也可以自定义。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">Random</span> <span class="variable">random</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">Random</span>();</span><br><span class="line">random.ints().limit(<span class="number">10</span>).sorted().forEach(System.out::println);</span><br></pre></td></tr></table></figure>

<h3 id="去重-distinct"><a href="#去重-distinct" class="headerlink" title="去重 distinct"></a>去重 distinct</h3><p><code>distinct</code> 方法通过流元素的 hashCode 和 equals 方法去除重复元素。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">Random</span> <span class="variable">random</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">Random</span>();</span><br><span class="line">random.ints().distinct().forEach(System.out::println);</span><br></pre></td></tr></table></figure>

<hr>
<h2 id="结束操作"><a href="#结束操作" class="headerlink" title="结束操作"></a>结束操作</h2><h3 id="迭代-forEach"><a href="#迭代-forEach" class="headerlink" title="迭代 forEach"></a>迭代 forEach</h3><p>结束操作： <code>forEach</code> 迭代流中的每个数据，即对每个数据进行最后的处理（比如保存到数据库中或打印）。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 输出 10 个随机数 </span></span><br><span class="line"><span class="type">Random</span> <span class="variable">random</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">Random</span>();</span><br><span class="line">random.ints().limit(<span class="number">10</span>).forEach(System.out::println);</span><br></pre></td></tr></table></figure>

<blockquote>
<p>不要用 forEach 方法收集元素！stream 并行执行会损害正确性和效率，使用下方操作。</p>
</blockquote>
<h3 id="聚合-Collectors"><a href="#聚合-Collectors" class="headerlink" title="聚合 Collectors"></a>聚合 Collectors</h3><p>结束操作：<code>Collectors</code> 类实现了归约操作，例如将流转换成集合和聚合元素，可用于返回列表或字符串。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// Stream 转化为 List</span></span><br><span class="line">List&lt;String&gt;strings = Arrays.asList(<span class="string">&quot;abc&quot;</span>, <span class="string">&quot;&quot;</span>, <span class="string">&quot;bc&quot;</span>, <span class="string">&quot;efg&quot;</span>, <span class="string">&quot;abcd&quot;</span>,<span class="string">&quot;&quot;</span>, <span class="string">&quot;jkl&quot;</span>);</span><br><span class="line">List&lt;String&gt; filtered = strings.stream()</span><br><span class="line">                               .filter(string -&gt; !string.isEmpty())</span><br><span class="line">                               .collect(Collectors.toList()); </span><br><span class="line">System.out.println(<span class="string">&quot;筛选列表: &quot;</span> + filtered);</span><br><span class="line"></span><br><span class="line"><span class="comment">// Stream 转化为 String</span></span><br><span class="line"><span class="type">String</span> <span class="variable">mergedString</span> <span class="operator">=</span> strings.stream()</span><br><span class="line">                             .filter(string -&gt; !string.isEmpty())</span><br><span class="line">                             .collect(Collectors.joining(<span class="string">&quot;, &quot;</span>));</span><br><span class="line">System.out.println(<span class="string">&quot;合并字符串: &quot;</span> + mergedString);</span><br></pre></td></tr></table></figure>


<h3 id="统计-SummaryStatistics"><a href="#统计-SummaryStatistics" class="headerlink" title="统计 SummaryStatistics"></a>统计 SummaryStatistics</h3><p>结束操作：收集最终产生的统计结果，它们主要用于 int、double、long 等基本类型上。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">List&lt;Integer&gt; numbers = Arrays.asList(<span class="number">3</span>, <span class="number">2</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">7</span>, <span class="number">3</span>, <span class="number">5</span>);</span><br><span class="line"> </span><br><span class="line"><span class="type">IntSummaryStatistics</span> <span class="variable">stats</span> <span class="operator">=</span> numbers.stream()</span><br><span class="line">                                    .mapToInt((x) -&gt; x)</span><br><span class="line">                                    .summaryStatistics();</span><br><span class="line"> </span><br><span class="line">System.out.println(<span class="string">&quot;列表中最大的数 : &quot;</span> + stats.getMax());</span><br><span class="line">System.out.println(<span class="string">&quot;列表中最小的数 : &quot;</span> + stats.getMin());</span><br><span class="line">System.out.println(<span class="string">&quot;所有数之和 : &quot;</span> + stats.getSum());</span><br><span class="line">System.out.println(<span class="string">&quot;平均数 : &quot;</span> + stats.getAverage());</span><br></pre></td></tr></table></figure>
 
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        <i class="ri-search-line"></i>
      </a>
      
      
      <a class="nav-item-link" target="_blank" href="/atom.xml" title="RSS Feed">
        <i class="ri-rss-line"></i>
      </a>
      
    </li>
  </ul>
</nav>
<div class="search-form-wrap">
  <div class="local-search local-search-plugin">
  <input type="search" id="local-search-input" class="local-search-input" placeholder="Search...">
  <div id="local-search-result" class="local-search-result"></div>
</div>
</div>
    </aside>
    <div id="mask"></div>

<!-- #reward -->
<div id="reward">
  <span class="close"><i class="ri-close-line"></i></span>
  <p class="reward-p"><i class="ri-cup-line"></i>请我喝杯咖啡吧~</p>
  <div class="reward-box">
    
    <div class="reward-item">
      <img class="reward-img" src="/images/alipay.jpg">
      <span class="reward-type">支付宝</span>
    </div>
    
    
    <div class="reward-item">
      <img class="reward-img" src="/images/wechat.jpg">
      <span class="reward-type">微信</span>
    </div>
    
  </div>
</div>
    
<script src="/js/jquery-3.6.0.min.js"></script>
 
<script src="/js/lazyload.min.js"></script>

<!-- Tocbot -->
 
<script src="/js/tocbot.min.js"></script>

<script>
  tocbot.init({
    tocSelector: ".tocbot",
    contentSelector: ".article-entry",
    headingSelector: "h1, h2, h3, h4, h5, h6",
    hasInnerContainers: true,
    scrollSmooth: true,
    scrollContainer: "main",
    positionFixedSelector: ".tocbot",
    positionFixedClass: "is-position-fixed",
    fixedSidebarOffset: "auto",
  });
</script>

<script src="https://cdn.staticfile.org/jquery-modal/0.9.2/jquery.modal.min.js"></script>
<link
  rel="stylesheet"
  href="https://cdn.staticfile.org/jquery-modal/0.9.2/jquery.modal.min.css"
/>
<script src="https://cdn.staticfile.org/justifiedGallery/3.8.1/js/jquery.justifiedGallery.min.js"></script>

<script src="/dist/main.js"></script>

<!-- ImageViewer -->
 <!-- Root element of PhotoSwipe. Must have class pswp. -->
<div class="pswp" tabindex="-1" role="dialog" aria-hidden="true">

    <!-- Background of PhotoSwipe. 
         It's a separate element as animating opacity is faster than rgba(). -->
    <div class="pswp__bg"></div>

    <!-- Slides wrapper with overflow:hidden. -->
    <div class="pswp__scroll-wrap">

        <!-- Container that holds slides. 
            PhotoSwipe keeps only 3 of them in the DOM to save memory.
            Don't modify these 3 pswp__item elements, data is added later on. -->
        <div class="pswp__container">
            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
        </div>

        <!-- Default (PhotoSwipeUI_Default) interface on top of sliding area. Can be changed. -->
        <div class="pswp__ui pswp__ui--hidden">

            <div class="pswp__top-bar">

                <!--  Controls are self-explanatory. Order can be changed. -->

                <div class="pswp__counter"></div>

                <button class="pswp__button pswp__button--close" title="Close (Esc)"></button>

                <button class="pswp__button pswp__button--share" style="display:none" title="Share"></button>

                <button class="pswp__button pswp__button--fs" title="Toggle fullscreen"></button>

                <button class="pswp__button pswp__button--zoom" title="Zoom in/out"></button>

                <!-- Preloader demo http://codepen.io/dimsemenov/pen/yyBWoR -->
                <!-- element will get class pswp__preloader--active when preloader is running -->
                <div class="pswp__preloader">
                    <div class="pswp__preloader__icn">
                        <div class="pswp__preloader__cut">
                            <div class="pswp__preloader__donut"></div>
                        </div>
                    </div>
                </div>
            </div>

            <div class="pswp__share-modal pswp__share-modal--hidden pswp__single-tap">
                <div class="pswp__share-tooltip"></div>
            </div>

            <button class="pswp__button pswp__button--arrow--left" title="Previous (arrow left)">
            </button>

            <button class="pswp__button pswp__button--arrow--right" title="Next (arrow right)">
            </button>

            <div class="pswp__caption">
                <div class="pswp__caption__center"></div>
            </div>

        </div>

    </div>

</div>

<link rel="stylesheet" href="https://cdn.staticfile.org/photoswipe/4.1.3/photoswipe.min.css">
<link rel="stylesheet" href="https://cdn.staticfile.org/photoswipe/4.1.3/default-skin/default-skin.min.css">
<script src="https://cdn.staticfile.org/photoswipe/4.1.3/photoswipe.min.js"></script>
<script src="https://cdn.staticfile.org/photoswipe/4.1.3/photoswipe-ui-default.min.js"></script>

<script>
    function viewer_init() {
        let pswpElement = document.querySelectorAll('.pswp')[0];
        let $imgArr = document.querySelectorAll(('.article-entry img:not(.reward-img)'))

        $imgArr.forEach(($em, i) => {
            $em.onclick = () => {
                // slider展开状态
                // todo: 这样不好，后面改成状态
                if (document.querySelector('.left-col.show')) return
                let items = []
                $imgArr.forEach(($em2, i2) => {
                    let img = $em2.getAttribute('data-idx', i2)
                    let src = $em2.getAttribute('data-target') || $em2.getAttribute('src')
                    let title = $em2.getAttribute('alt')
                    // 获得原图尺寸
                    const image = new Image()
                    image.src = src
                    items.push({
                        src: src,
                        w: image.width || $em2.width,
                        h: image.height || $em2.height,
                        title: title
                    })
                })
                var gallery = new PhotoSwipe(pswpElement, PhotoSwipeUI_Default, items, {
                    index: parseInt(i)
                });
                gallery.init()
            }
        })
    }
    viewer_init()
</script> 
<!-- MathJax -->

<!-- Katex -->

<!-- busuanzi  -->
 
<script src="/js/busuanzi-2.3.pure.min.js"></script>
 
<!-- ClickLove -->

<!-- ClickBoom1 -->

<script src="https://cdn.staticfile.org/animejs/3.2.1/anime.min.js"></script>

<script src="/js/clickBoom1.js"></script>
 
<!-- ClickBoom2 -->

<!-- CodeCopy -->
 
<link rel="stylesheet" href="/css/clipboard.css">
 <script src="https://cdn.staticfile.org/clipboard.js/2.0.10/clipboard.min.js"></script>
<script>
  function wait(callback, seconds) {
    var timelag = null;
    timelag = window.setTimeout(callback, seconds);
  }
  !function (e, t, a) {
    var initCopyCode = function(){
      var copyHtml = '';
      copyHtml += '<button class="btn-copy" data-clipboard-snippet="">';
      copyHtml += '<i class="ri-file-copy-2-line"></i><span>COPY</span>';
      copyHtml += '</button>';
      $(".highlight .code pre").before(copyHtml);
      $(".article pre code").before(copyHtml);
      var clipboard = new ClipboardJS('.btn-copy', {
        target: function(trigger) {
          return trigger.nextElementSibling;
        }
      });
      clipboard.on('success', function(e) {
        let $btn = $(e.trigger);
        $btn.addClass('copied');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-checkbox-circle-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPIED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-checkbox-circle-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
      clipboard.on('error', function(e) {
        e.clearSelection();
        let $btn = $(e.trigger);
        $btn.addClass('copy-failed');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-time-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPY FAILED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-time-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
    }
    initCopyCode();
  }(window, document);
</script>
 
<!-- CanvasBackground -->
 
<script src="/js/dz.js"></script>
 
<script>
  if (window.mermaid) {
    mermaid.initialize({ theme: "forest" });
  }
</script>


    
    

  </div>
</body>

</html>